Have you checked the CPS (Crank Position Sensor)? They have a habit of going with no warning, no spark and Jaguar want a small fortune for them. However you can get them on ebay for ~£15, i did for my S Type 3.0 which uses the same as most petrol V8s/V6s, the Ford Thunderbird and Lincoln LS of similar years.
They're a bit of a faff to change but a relatively cheap and easy fix, best approached from underneath so a good jack and axle stands are needed.